The present invention relates to electrical circuits, and in particular, to a circuit and method for regulating discharge of a capacitor.
Capacitors of several Farads to thousands of Farads are being used or are proposed to be used as the primary energy source of a variety of systems—from toys, cell phones, to electric automobiles. The intent is to replace rechargeable batteries with capacitors in the belief that the system will be lighter weight, longer lasting, and offer extremely rapid recharge cycles.
A problem exists in how to regulate energy transfer, such as voltage and current, from an energy storage capacitor to a load. For example, regulating a voltage and current from a capacitor to a load is different than regulating a voltage and current from a battery to a load. Typically, energy from a battery is coupled to a load through a regulator, which may receive a voltage and current from the battery and provide a constant output voltage to the load across a range of load currents. The voltage at the output of the regulator may be higher or lower than the battery voltage, and the load currents may be higher or lower than the current drawn from the battery under the condition that the energy transferred from the battery is greater than the energy provided to the load (the difference is a loss and depends on the efficiency of the regulator). A battery typically outputs a constant voltage and a may provide current according to the battery's rating. The voltage on a battery may drop off at the end of the battery's charge, but otherwise remains relatively stable. However, batteries typically have very long charge cycles and only output voltages at the battery's rated value.
Capacitors, unlike batteries, may be charged quickly and may store a wide range of voltages. The amount of current a capacitor may provide depends on how much charge the capacitor stores. Larger value capacitors store larger amounts of charge, which may be used to provide current to a load. However, also unlike batteries, capacitors may have a significant decrease in voltage as charge is transferred to a load. Accordingly, as charge is provided a load, the voltage on the capacitor may drop significantly. If the load is an electronic device, the voltage may quickly fall below the devices minimum operating level while a significant amount of energy remains on the capacitor.